Modern wireless communications standards such as those for Long Term Evolution (LTE) and LTE advanced dictate how signals should be transmitted and received from a wireless communications device. In doing so, these standards place a number of requirements on a wireless communications device, such as output power requirements, spectral masking requirements, and filtering requirements for receive signals that in turn dictate the physical structure of the device. As wireless communications standards continue to advance, these requirements grow in size and complexity. For example, due to the increasing number of bands supported by modern wireless communications standards along with the use of carrier aggregation and multiple-input-multiple-output (MIMO), a wireless communications device must include filters that have both high selectivity and high bandwidth. Often, these requirements are difficult to achieve without significantly increasing the size, cost, and complexity of the wireless communications device.
To address the stringent filtering requirements imposed by modern wireless communications standards, acoustic filters have been increasingly used. While acoustic filters often outperform their lumped element counterparts in terms of selectivity and quality factor, the bandwidth of acoustic filters is highly limited due to the relatively low electromechanical coupling that is physically achievable. Accordingly, lumped element filters are still required for high bandwidth applications.
For purposes of illustration, FIG. 1 is a functional schematic showing a conventional lumped element diplexer 10. The conventional diplexer 10 includes a first port 12, a second port 14, and a third port 16. A first capacitor C1 is coupled in series between the first port 12 and the second port 14. A second capacitor C2 is coupled in series between the first port 12 and the third port 16. A first resonator R1 is coupled in series with a third capacitor C3 between the first port 12 and ground. The first resonator R1 includes a first resonator capacitor CR1 coupled in parallel with a first resonator inductor LR1 between the third capacitor C3 and ground. A second resonator R2 is coupled in series with a fourth capacitor C4 between the second port 14 and ground such that the first capacitor C1 is coupled between the first resonator R1 and the second resonator R2. The second resonator R2 includes a second resonator capacitor CR2 coupled in parallel with a second resonator inductor LR2 between the fourth capacitor C4 and ground. A third resonator R3 is coupled in series with a fifth capacitor C5 between the first port 12 and ground, such that the third resonator R3 is in parallel with the first resonator R1. The third resonator R3 includes a third resonator capacitor CR3 coupled in parallel with a third resonator inductor LR3 between the fifth capacitor C5 and ground. A fourth resonator R4 is coupled in series with a sixth capacitor C6 between the third port 16 and ground such that the second capacitor C2 is coupled between the third resonator R3 and the fourth resonator R4. The fourth resonator R4 includes a fourth resonator capacitor CR4 coupled in parallel with a fourth resonator inductor LR4 between the sixth capacitor C6 and ground.
While only two resonators are shown in each signal path for purposes of illustration, conventional designs have included any number of resonators to provide a desired filter response. To achieve one or more desired performance characteristics (e.g., quality factor, bandwidth, selectivity), it is often desirable to provide coupling (i.e., inductive coupling or mutual inductance) between various ones of the first resonator inductor LR1, the second resonator inductor LR2, the third resonator inductor LR3, and the fourth resonator inductor LR4. Said coupling may be used to obtain a desired bandwidth of the conventional diplexer 10, provide cancellation of signals between signal paths, or otherwise tune the operation of the diplexer. Coupling is expressed by a coupling factor k, also known as a coupling coefficient, which is a value between negative one and one (−1≤k<1) representing both the magnitude and direction of the coupling. The desired level of coupling varies between the different resonator inductors LR. For example, it may be desirable to provide high coupling between some of the resonator inductors LR such that a coupling factor between the resonator inductors LR is greater than 0.4, provide moderate coupling between some of the resonator inductors LR such that a coupling factor between the resonator inductors LR is between 0.1 and 0.4, and provide low or no coupling between other ones of the resonator inductors LR such that a coupling factor between the resonator inductors LR is less than 0.1.
Using conventional inductor structures such as planar inductors and “figure 8” inductors, the aforementioned desired coupling factors between resonator inductors LR are very difficult to achieve. This is due to the fact that conventional inductor structures provide a relatively large magnetic field perpendicular to a plane on which other inductor structures are located with little to no cancellation thereof. Accordingly, coupling between nearby conventional inductor structures is largely dictated by the space between them, often requiring very large distances between inductor structures to obtain moderate, low, or no electromagnetic coupling. This is often impractical or impossible in wireless communications devices where space is highly limited. Further, the performance (e.g., quality factor) of conventional inductor structures is often quite low, making them unsuitable for many applications such as the stringent filtering discussed above.
In light of the above, there is a need for improved inductor structures for providing coupled inductors with desired coupling factors in a minimal form factor and with high performance.